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If you were a terrorist looking for weapons-grade nuclear material in America, the Lawrence Livermore National Laboratory might be a good place to start. At the core of the nuclear-weapons research facility about an hour's drive from San Francisco stands the "Superblock," a collection of buildings surrounded by multi-story steel-mesh fencing, a no-man's-land, electronic security gear, armed guards and cables to prevent a helicopter landing on the roof. These defenses are in place largely to protect Building 332, a repository for roughly 2,000 pounds of deadly plutonium and volatile, weapons-grade uranium  enough fissile material to build at least 300 nuclear weapons. But a recent simulated terror attack tested those defenses, and sources tell TIME that the results were not reassuring.

One night several weeks ago, according to TIME's sources, a commando team
posing as terrorists attacked and penetrated the lab, quickly overpowering
its defenses to reach its "objective"  a mock payload of fissile material. The exercise highlighted a number of serious security
shortcomings at Livermore, sources say, including the failure of a
hydraulic system essential to operating an extremely lethal Gatling
gun that protects the facility. Experts contacted by TIME  including
congressional staff from both parties informed of the episode, and experts
personally familiar with safeguards at Livermore  all said that the test amounts to an embarrassment to those responsible for securing the nation's nuclear facilities, and that it required immediate steps to correct what some called the most dangerous security weaknesses ever found at the lab.

Energy Secretary Samuel Bodman was quickly informed of the episode, along with other senior officials in the U.S. nuclear and national security apparatus. "People who know about this are very concerned; they are not happy," said one senior congressional aide.

"It is essential to prevent terrorists from accessing nuclear materials at Livermore," said Danielle Brian, the executive director of the Project on Government Oversight, an independent nonprofit that recently issued a study of the lab's security. "Suicidal terrorists would not need to steal the fissile material, they could simply detonate it as part of an improvised nuclear device right on the spot." Some 7 million people live within a 50-mile radius of the laboratory  a fact that has prompted at least one panel of experts to recommend moving its nuclear-weapons material elsewhere.

According to a former senior officer familiar with the details of
security at Livermore, simulated attacks are staged approximately every 12 months. The attack team's objective is usually to penetrate the "Superblock," after which the attackers are timed to determine whether they can hold their ground long enough to construct a crude "dirty bomb" that could, in theory, be detonated immediately, or can buy themselves enough time to fabricate a rudimentary nuclear device, approximating the destructive power of the low-yield weapons dropped on Hiroshima and Nagasaki in 1945. A third option in the simulation is for the attackers to abscond with the nuclear material into the heavily populated San Francisco Bay area.

The security flaws exposed in the recent test could exacerbate public opposition to nuclear weapons material being stored at Livermore, which is located near a major highway interchange, atop a vital agricultural irrigation canal and within a mile of two elementary schools, a preschool, a middle school and a senior center. In 2005 the Energy Department approved the doubling of the amount of plutonium stored at Livermore, less than five months after a scientific panel recommended, for security reasons, that nearly all of it be moved to a safer, more remote site.

"The fissile material simply cannot be made safe and secure," says
Marylia Kelley, executive director of Tri-Valley CARES, a Livermore nuclear weapons watchdog group. "We in the community, which has 81,000 people, want to get rid of the plutonium and highly enriched uranium as soon as possible."

The alleged failure of Livermore's truck-mounted Gatling guns could
also draw heavy criticism. Those weapons have long been controversial
because they can fire 4000 rounds a minute and kill a person more than a mile away, raising fears among local residents about what might happen if the guns were ever discharged. The weapons are also supposed to be tested on a regular basis, and the reason for their reported failure remains
unclear.

Many critics have also argued that the entire process of conducting "force-on-force" simulations at Livermore is flawed because the exercise does not adequately approximate conditions that would pertain during a real attack. The defenders are always given advance notice of the simulations, which usually occur at night or on weekends, when few of the facility's
thousands of staff are present. As a result, there is no simulation of
the hostage-taking that might occur if the lab were attacked during business hours. The absence of most regular employees also means that
defenders do not have to worry about directing their fire to avoid
innocent victims, many of whom might be present during an actual
attack.

Finally, nothing in the "force-on-force" exercises simulates the danger posed by Livermore being situated beneath the flight path to several nearby
airports. "If a plane ever tried to fly into the lab," says Tri-Valley CARE'S Kelley, "no one has ever explained how it would be stopped."

As for the Department of Energy, in a press release issued last Friday
referring to the recent force-on-force exercise at Livermore, it claimed that an inspection team sent to the site after the simulation had noted both "several very positive areas" and "other areas requiring corrective action."

"We do not believe the [nuclear] materials at Livermore are at risk, and we do believe that security is strong," a DOE spokesperson told TIME. "But we're also interested in examining any deficiencies, which is the purpose of these routine exercises."